<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <script src="https://cdn.bootcss.com/jquery/3.4.1/jquery.min.js"></script>
    <title>Document</title>
</head>

<body>
    <div>
        <input type="file" id="uploadImg" onchange="xmTanUploadImg(this)">
        <img src="" alt="" id="selectImg">
    </div>
    <script>
        function xmTanUploadImg(obj) {
            var file = obj.files[0];
            var reader = new FileReader();
            reader.readAsDataURL(file);
            reader.onload = function (e) {    //成功读取文件
                var img = document.getElementById("selectImg");
                img.src = e.target.result;   //或 img.src = this.result / e.target == this
            };
        }
    </script>
</body>

</html>